Kaolin is a naturally occurring, relatively fine, white clay mineral which may be generally described as a hydrated aluminum silicate (Al2O3.2SiO2.2H2O). The structure of kaolin is principally one octahedral Al(OH)3 sheet covalently bonded with one tetrahedral SiO4 sheet to form a 1:1 layer. Ideally, this 1:1 layer is electrically neutral. Adjacent layers are held together primarily by hydrogen bonding between the basal oxygen atoms of the tetrahedral sheet and the hydroxyls of the surface plane of the adjacent octahedral sheet.
The ideal structural formula of kaolin can be represented as Al2Si2O5(OH)4. After purification and beneficiation, kaolin is widely used as a filler and pigment in various materials, such as rubber and resins, and in various coatings, such as paints and coatings for paper.
The use of kaolin in paper coatings serves, for example, to improve brightness, color, gloss, smoothness, opacity, printability and uniformity of appearance of the coated paper. As a filler in paper formulations, kaolin is used to extend fiber and reduce cost, and to improve opacity, brightness and other desirable characteristics of the filled paper product.
Kaolin clay is naturally hydrous and may contain as much as 13.95% water in the structure in the form of hydroxyl groups. Examples of hydrous kaolin clay are the products marketed by Thiele Kaolin Company (Sandersville, Ga.) under the trademarks Kaofine 90 and Kaolux. These products have not been subjected to a calcination step.
Calcined kaolin is another type of kaolin and is obtained by heating (i.e., calcining) beneficiated kaolin clay at temperatures of at least 550° C. The calcination step dehydroxylates and converts the kaolin into a noncrystalline aluminosilicate phase. (The term “dehydroxylates” refers to the removal of structural hydroxyl groups from the kaolin in the form of water vapor.) The smaller particles of the feed clay are aggregated by calcination, and this aggregation increases the original volume of the kaolin and gives the calcined kaolin a “fluffy” appearance. Particle aggregation increases the light scattering characteristics of the kaolin (as compared to non-calcined kaolin) and, therefore, contributes to a high degree of opacity and insulating properties to a coated paper.
In addition, calcination increases the brightness of kaolin. An example of calcined kaolin clay is the product marketed by Thiele Kaolin Company under the trademark KAOCAL. The high brightness of the calcined clay is partly due to the removal of organic material and partly due to the mobilization of the impurity phases in the amorphous network at elevated temperatures. The brightness can also be improved through pre-calcination beneficiation processes such as magnetic separation, froth flotation, selective flocculation and chemical leaching.
Both hydrous and calcined kaolin clay products are useful in coating compositions for conventional printing applications such as offset, rotogravure, letterpress and flexographic. However, without substantial mechanical and/or chemical modifications, conventional hydrous and calcined kaolin clay products are not useful in coating compositions for ink jet printing applications.
In an ink jet printing process, uniformly shaped tiny droplets of aqueous or solvent based dye solutions are ejected from a nozzle onto a substrate. There are two primary types of ink jet printing—continuous ink jet printing and drop on demand ink jet printing (DOD). The continuous ink jet is used in high speed printing such as addressing, personalization, coding and high resolution color printing such as proofing. The DOD ink jet is mainly used in home, office and wide format printing.
Common DOD ink jet printers are the thermal ink jet printer and the piezoelectric printer. In the thermal (or bubble jet) process, ink is heated and vaporized periodically with a heating element connected to the digital data to generate bubbles. Since the volume of the ink increases during vaporization, the ink is forced out of the nozzle in the form of a drop which is deposited on the paper.
In the piezoelectric process, the drop is generated by pressure using a piezoelectric crystal instead of heat as in the thermal method. The piezoelectric materials exhibit the “piezo-electric effect”; that is, the materials undergo distortion when an electric field is applied. The piezoelectric crystal mounted behind the nozzle expands and shrinks when an electrical pulse is applied, followed by displacement of drops from the nozzle. The piezoelectric printer has several advantages (e.g., a more controlled and higher rate of drop production and long head life) over the thermal printer.
Ink jet printing requires special paper for achieving high quality images due to the nature of the inks used and the design of the printhead. Most of these inks are anionic and principally consist of water and a water soluble solvent. Inks are jetted from a series of very small orifices, each approximately 10-70 μm in diameter, to specified positions on a media to create an image. Multipurpose plain paper is unsuitable for good quality ink jet printing since that type of paper causes numerous quality issues such as feathering, wicking, color bleeding, low color density, strike-through and cockle/curl. Consequently, ink jet papers are commonly coated with special ink receptive layers which are formulated to provide good print quality and adequate ink drying/absorption.
Amorphous silica (such as silica gel) is a commonly used pigment for the matte grade ink jet coating applications. The high surface area and porous silica pigment provides high porosity coatings for quick absorption of ink solvent and rapid ink drying time. However, silica gel is expensive and can only be made down at very low solids. For example, most silica gels can be made down at only 15-18% solids which may result in low coating solids.
Several non-silica based pigments for ink jet paper coating applications are known in the industry. For example, heat aged-precipitated calcium carbonate is disclosed in Donigan et al. U.S. Pat. No. 5,643,631.
Chen et al. PCT International Publication No. WO 98/36029 and Chen et al. U.S. Pat. No. 6,150,289 disclose a coating composition comprising calcined clay, a cationic polymer, polyvinyl alcohol, a latex binder and optionally a cross-linking agent.
Londo et al. U.S. Pat. No. 5,997,625 discloses a coating composition comprising a fine particle hydrous clay, a caustic leached calcined clay and a porous mineral (zeolite).
Malla and Devisetti U.S. Pat. No. 6,610,136 discloses aggregated mineral pigments having a high surface area and low light scattering and useful in coating and filling compositions for ink jet printing media.
All of the above non-silica based pigments are primarily designed for matte grade ink jet coated paper. However, in most of the photographic and high end ink jet printing applications, a glossy coated paper is preferred. Currently, there are two types of glossy coatings: (1) a swellable polymer coating and (2) a microporous coating.
In a swellable polymer coating, the drying of ink is slow and involves diffusion of water molecules into the polymer matrix and swelling of the polymer matrix. Polymers such as polysaccharides (cellulose derivatives), gelatins, poly(vinyl alcohol), poly(vinyl pyrrolidone) and poly(ethylene oxide) are used in swellable coatings. On the other hand, ink drying is relatively fast in a microporous coating which occurs due to water absorption into the pore structures of the coating and base paper by capillary action. High surface area and very fine particle pigments such as alumina, aluminum hydroxides, fumed silica, and colloidal silica are the pigments of choice for glossy coatings.
Berube et al. U.S. Pat. No. 6,585,822 discloses the use of fine particle kaolin clay as a gloss coating on a paper pre-coated with a layer of a microporous ink jet coating pigment comprising a mixture of hydrous kaolin clay, caustic leached calcined kaolin clay and a zeolitic molecular sieve. The glossy pigment coating requires that the paper be pre-coated with a highly absorbent coating layer.
The above mentioned pigments can be very expensive, difficult to handle or do not meet the performance requirements. Thus, there is a need in the industry for a cost effective mineral pigment that meets the performance requirements for glossy ink jet printing applications.